When someone has too much insulin in their blood for a long time, it blocks the brain's ability to respond to leptin - the hormone that tells us we're full - so the person keeps feeling hungry and eating even though their body already has enough energy stored.

The study tested early overfeeding in baby rats, which caused them to develop high insulin levels and later become leptin resistant in their brains. This supports the idea that high insulin can cause leptin resistance, though the study didn't directly test injecting insulin as the intervention.